1. Technical Field
The present invention relates to the field of interference measurements. In particular, the invention relates to a method of performing interference measurements in a multiple access wireless communications system utilizing a carrier sensing technique for monitoring during a sensing period if a carrier is idle. The invention further relates to a transmitting unit implementing such a method and a wireless communications system comprising this transmitting unit.
2. Discussion of the Prior Art
Wireless communications systems require access schemes which define how individual system components may share available system resources like frequency bands. Multiple access control protocols are techniques that allow individual system components to temporarily access available system resources on a demand basis.
Examples of multiple access control protocols are Carrier Sense Multiple Access (CSMA) schemes like CSMA with Collision Detection (CSMA/CD) and CSMA with Collision Avoidance (CSMA/CA). CSMA is based on a carrier sensing scheme which allows a plurality of system components to share one or more carriers for the purpose of transmitting information. Only when a system component “senses” that a carrier is idle, i.e., that there is no traffic on the carrier, can it start its transmission. Once a system component transmits on the carrier, no other system component can transmit until the carrier becomes idle again. In accordance with the multiple access aspect, many system components simultaneously attempt to transmit, and each system component first listens (or senses) to detect any possible collisions. The period of time during which a system component senses a carrier prior to transmission (sensing period) may be constant for all system components or may be individually determined.
As has become apparent from the above, CSMA is an efficient method for sharing one or more carriers among individual components of a wireless communications system. However, specific frequency bands are often shared not only among individual components of one and the same wireless communications system, but among a plurality of wireless communications systems and further users like radar systems and the like. For example the ISM frequency band (2,4 GHz) is concurrently used by the wireless communications systems IEEE 802.11b and Bluetooth. Another example is the 5 GHz frequency band which in Europe is used by the High PErformance Radio Local Area Network type 2 (HIPERLAN/2, or simply H/2) and radar systems. In the near future further wireless communications systems like the IEEE 802.11a system, which uses CSMA/CA, might operate in the 5 GHz frequency band also.
If a plurality of different systems has to share one and the same frequency band, collisions among these systems have to be prevented. To that end, Dynamic Frequency Selection (DFS) as required for example for the 5 GHz frequency band by the European Radio Communications Committee in the decision ERC/DEC/(99)23 may be implemented. DFS incorporates dynamic frequency adaption to local interference conditions. The task of DFS is to perform interference measurements and to select the least interfered carrier for transmission. DFS thus allows to avoid co-channel operation with other systems. A common implementation of DFS is to periodically measure interference on the carrier used for transmission and all other available carriers and to automatically select a new carrier in the case the currently used carrier is suddenly disturbed by an interferer and a less disturbed carrier is available. Consequently, DFS guarantees a high transmission quality on the one hand and avoids interference with other systems on the other hand.
Any kind of DFS for CSMA systems like IEEE 802.11a is currently neither supported nor standardized. One reason therefore is that the CSMA mechanism is difficult to combine with longterm interference measurements. This is a result of the fact that the interference measurement periods cannot be longer than the sensing period since otherwise an interference measurement for a carrier performed by a first transmitting unit could be interpreted as idle time by a second transmitting unit in the close vicinity. The second transmitting unit may thus start to transmit on this carrier, which will disturb the interference measurement performed by the first transmitting unit. The measuring first transmitting unit may for example interpret the transmission of the second transmitting unit as radar interference and may hence skip this carrier, which is both unwanted and unnecessary.
Such a misinterpretation of the measuring transmitting unit can be avoided if the measuring unit can decode the interference signal. When the measuring unit thus detects that the interference is caused by another component of the same wireless communications system, it will not interpret the interference as radar interference. This helps to reduce the false alarm probability, but it cannot avoid that the interference measurement is interrupted for an unpredictable period of time, which renders the interference measurement rather unreliable.
In order to avoid the interpretation of the measurement period as idle time one could also think about extending the sensing period or decreasing the measurement period such that the measuring period does not exceed the sensing period. The sensing period, however, cannot be significantly extended without decreasing the transmission capacity. On the other hand, the measurement period cannot be reduced just as one likes without decreasing the probability of detecting rare interferences like periodical interferences with a comparatively large interval between two subsequent interferences, such as radar signals.
One object of the present invention is to provide an improved method of performing an interference measurement in a multiple access wireless communications system utilizing a carrier sensing technique.
A further object of the invention is to provide a transmitting unit for a wireless communications system implementing this method and a wireless communications system comprising such a transmitting unit.